Piezoelectric crystal apparatus



Dec. 10, 1940. J. M. WOLFSKILL PIEZOELECTRIC CRYSTAL APPARATUS Filed Jan. 11, 1940 2 Sheets-Sheet 2 -/a /o 20 3a 4a 50 60 Ja Ja 9o flay/e FO EIf/OH Jam/1141127 jag/"ea:

J/M/W Patented Dec. 10, 1040 UNITED STATES PATENT OFFICE PIEZOELECTBIC CRYSTAL APPARATUS John M. Woifskill. Erie, Pa assignor to Bliley Electric Company, Erie, Pa. a partnership composed of F. Dawson Bliiey and Charles Collman Application January ll, 1940, Serial No. 313,462 14 Claims. (Cl. 171-327) This invention relates to a variable frequency tal controlled oscillators over the self-excited piezoelectric crystal element and more partictype are well known to the art, and there was ularly to a method of so cutting the crystal eletherefore a need for a method of obtaining a i ment from the crystal as to produce a variable much wider continuous frequency variation withfrequency characteristic. out sacrificing any of the advantages of crystal 5 One of the main objects of this invention is control. to obtain a wide continuous frequency variation This invention relates to a method of cutting with a crystal which is of substantially constant and grinding a quartz crystal as to enable a wide thickness. continuous frequency range to be obtained in a Another object of the invention is to provide a single crystal. In general, the method consists l0 piezoelectric element with.which a wide freof making the crystal in the form of a section quency variation may be obtained. By wide freof a cylinder such that by moving from one end quency variation is meant up to 100 kilocycles or of the section to the other. the angle of the eleseveral hundred kilocycles. mental sections to the optic axis varies continu- A further object is to provide a piezoelectric ously. The axis of the cylinder from which the 15 crystal with which a wide continuous frequency section is cut is made substantially parallel to variation may be obtained. the electric axis, and the angle to the optic axis Another object is to provide a wide range variof the elemental sections can vary anywhere from able frequency quartz element whose thickness 0 to plus or minus 90 degrees, depending on the is constant but whose frequency thickness coefdesired frequency variation. go ficient varies along the length of the crystal. From some of the curves described in detail The need for a wide range variable frequency further in the specification, it is seen that as the quartz crystal is becoming more and more imangle at which the crystal is cut to the optic axis portant in communication art, and this is paris varied, the thickness coefficient varies accordticularly true in the amateur field where it is ing to the curve, and since the frequency for any 25 desirable to change frequency rapidly from one given thickness of crystal is dependent on this portion of the band to another to avoid interthickness coefficient from the simple relation ference from other stations. Various methods have been devised for varying the controlling so frequency, these include electron coupled and other self-excited oscillators as well as narrow the frequency also varies. It is then possible to range variable frequency crystal controlled osobtain any desired frequency variation along cillators. such a section of a cylinder by properly choosing The variation obtainable with a single quartz the limits of angle of the tangent to the quartz crystal by present methods known to the art, section. From the limits of angle and the length as however, is relatively small, and it has genof the crystal section, the required radius of the erally been conceded that the variation obtaincylindrical section may be computed. In the able by an air-gap type of holder is insuihclent use of a crystal of this type, one electrode natfor certain services. A method of continuously urally will have to assume the same curvature as 40 varying the frequency of a quartz crystal over a the section of crystal, and another movable elec- 4 relatively narrow frequency range is described in trode (which can be in the form of a flat narrow my Patent No. 2,079,540. This invention covers electrode or a roller) which is of necessity small a wedge type adjustable air-gap which is assoin dimension serves to excite the elemental crysciated with a quartz crystal, and the variation of tals. The crystal will be of a constant thickness 5 the gap controls the frequency variation of the along its length, and the frequency variation becrystal. tween the elements is obtained entirely by virtue Other methods of obtaining step frequency of the varying thickness coefiicient. variation include those of using a number of in- Further details of this invention are set forth dividual crystals connected with a tap switch-for in the following specification, the claims and the so rapid switching from one crystal to another. drawings in which briefly, Fig. 1 illustrates the so Such arrangements, however, do not have the manner in which the crystal element is cut from advantages of a self-excited oscillator because the mother crystal; Fig. 2 shows a crystal eleoften it is desirable to use a frequency or frement forming a section of a cylinder and two quencies somewhere between the frequencies of imaginary fiat crystal elements are positioned as the step crystals. The advantages of quartz crystangent thereto for purposes of explanation of this invention; Figs. 3, 4 and 5 illustrate examples of crystal elements cut in accordance with this invention; Figs. 6, 7 and 8 show forms of electrodes and holders that may be employed with the crystal elements described herein; Fig. 9 is a curve showing the relation between the temperature coefficient and the angle of rotation of a crystal element about an electric axis; and Fig. 10 is a curve illustrating the relation between the frequency thickness coefficient and the angular rotation about an electric axis.

Referring to Fig. l of the drawingsin detail reference numeral I designates the mother quartz crystal having the optic or Z axis, an electric or X axis and a mechanicalor Y axis designated therein as illustrated. The crystal element ll having the principal faces l2 and if of curved or bowed configuration so as to form sections or surfaces of a cylinder, is shown in dotted outline in the crystal III. In Fig. 2 a similar view of the crystal element II is shown with two substantially flat crystal elements H and i drawn in dotted outline tangent to the ends of crystal section I I. These two imaginary plane crystals I4 and I5 represent the limits of the angular variation of the elemental crystals which make up the bowed crystal ii. In choosing the section of the frequency thickness curve shown in Fig. 10, over which it is best to obtain angular variation, it is naturally desirable to keep the temperature coefficient as low aspossible and the activity of the crystal as high as possible. An angle of plus 30 degrees was selected for the end l6 and the crystal element was cut with its principal faces substantially parallel to the X axis and with the elemental section of said faces at the end ii at an angle of substantially 30 degrees with respect to the optic or Z axis; this angle, however, is decreased as far as it is necessary to obtain the desired frequency variation. It is of course obvious that these angles and positions are not the only ones that are desirable and that these are set forth here only to explain features of this invention which may be applied to other angles and different types of crystal cuts. From the curve Fig. 10, it is seen that the imaginary plane crystal I cut at plus 30 degrees has a frequency thickness coefficient of 0655x Consequently for a frequency of, for example, 3500 kilocycles, the crystal will have to be 0.0187" thick. Assuming that a variation of 100 kilocycles is to be obtained, the thickness coefficient of the other imaginary crystal I! at the other end I! of the section II will have to be equal to (K=FT) =0.0187 3600 kilocycles or 0.673x 10'. From the curve, I-"igurelO, it is seen that for a thickness coemcient of 0.673, theangle of the imaginary crystal ll tangent to the section I I will have to be approximately plus 17 degrees. In moving along the section, then, the angle of the elemental crystals to the optic axis will vary from plus 17 to plus 30 degrees or a total change of 13 degrees. In order to have a reasonable sized crystal and also a relatively large size movable electrode, the crystal is made one and onehalf inches long by three-fourths inch wide, however, of course, these dimensions are simply arbitrary and do not influence the performance of the crystal. I

Referring to Figure 3, it is seen that for a crystal element ll approximately one and one-halfinches long. the radius R of curvature required to obtain the angular variation of plus 17 degrees for angle 01 to plus 30 degrees forangle 0: of the tangents at ends I! and i6 respectively, is

degrees.

demonstrate the manner in which the dimensions and the proper curvature of the crystal are computed to obtain some'deflnite specified frequency variation. Referring to Fig. 10 showing the frequency thickness coefficient versus angular rotation curve, it is seen that the curve is practically a straight line from plus 10 degrees to minus 40 degrees. This means that the frequency variation along a crystal I l curved so that the tangents to the ends I8 and ll of the crystal make angles of plus 10 degrees for 01 and minus 40 degrees for 0: with the optic axis, is linear with angular rotation.

Theoretically, this would be the best range in which to work all crystals of this type. Unfortunately, however, the temperature coefficient of the crystal between the angles plus 10 degrees and minus 10 degrees is relativelyhigh as seen.

from Fig. 9, and as a result it is desirable to eliminate this portion of the curve. The crystal can be cut at angles between minus 20 degrees and minus 45 degrees and still maintain a fairly low temperature coefficient over the range. A crystal of this type may be conveniently used in the low drive circuit described and claimed inmy copending application Serial No. 313,461, filed January 11, 1940.

If it is desired to obtain a crystal which works on the straight portion of the frequency thick-' ness curve and thus obtain a very large frequency variation, of, for example 400 kilocycles, this can bedone by starting at an angle of minus 20 degrees. At this angle the frequency thickness coeflicient is 0.865x l0 and consequently for a 3500 kilocycle crystal, the thickness is 0.0247". The upper frequency limit, assuming the 400 kilocycle frequency variation is 3900 kilocycles and for-a crystal 0.0247" thick, the coefficient K- must be (K=FT) equal to 0.0247X'3900X10 On the frequency thickness coefficient curve, Fig. 10 this corresponds to an angle of minus 40 For computing the radius of curvature required for a crystal one and one-half inches-long, reference is made to Figure 4 and in this case the calculations are similar to those in the case of Fig. 3. The total angular variation of the'crystalshown in Fig. 4 is 20 degrees which means that the angle d=%(92-0i)=10. From this it is found that R is equal 4.32" and X=4.25, or in other words, the crystal is actually ground concave on one side by .070" and convex on the other side by substantially the same amount.

For a still lower drift over the frequency range, the crystal may be out between minus 30 degrees and minus 45 degrees as illustrated in Fig. 5. The drift at the minus 30 degree end i1 is about plus 36 cycles/10'/ centigrade, and decreases to about plus 8 cycles/10F centigrade at the 45 degree angle end 18 as seen by referring to the curve shown in Fig. 9. By choosing the two angular limits first for the crystal element the frequency range over which the frequency may be varied is automatically limited. The thickness required for a 3500 kilocycle crystal cut at minus 30 degrees is 0.0262". Using this same thickness and the frequency thickness coefficient of the minus 45 degree angle, the other extreme end 6 of the frequency range of this crystal will be 3720 kilocycles. From the calculationsgiven in Fig. 5, it is found that the radius R required on a crystal of this type is 5.74".

In the manufacture of such a crystal as described in this invention, certain manufacturing difliculties are encountered. When it is noticed that even in the crystal which gave a frequency variation of 400 kilocycles, the amount of curvature or concaving was only on the order of 0.070 inch, the obvious method of manufacture is to first slice a parallel blank at the angle half way between the two extreme tangent angles 01 and 02. By then using a grinding surface which takes the form of a cylinder, having thesame radius of curvature as desired on the crystal, the inner surface I8 of the crystal is ground concave. The opposite side of the crystal may be ground in the same manner by using the inside of the cylinder as a grinding surface or grinding it on a concave section of a cylinder. In doing this grinding, the crystal must be moved in such a manner that the end edges move parallel to the axis of the cylinder. It must also be ground in such a way that the thickness of the crystal, as measured along the radius R, remains constant.

Due to the odd shape of the crystal of this invention, it can be seen that a special holder is required to utilize all the advantages of the crystal. The holder may be made in various forms, several of which are shown in Figs. 6, 'l and 8. Fig. 6 shows the crystal ll mounted on a curved bottom electrode I9, curved convex to the same radius as the crystal and with a movable electrode 40 20 in the form of a cylinder which is made to move along the length of the crystal so as to excite a small elemental area as it moves along the length. Fig. 7 shows the same arrangement, but instead of a roller electrode, a flat movable nar- 45 row electrode 2| is used to excite the crystal. A small air gap may be left between the electrode 2| and the crystal surface to eliminate abrasion.

Fig. 8 shows another method of exciting the crystal and this makes use of two movable cylinders 50 22 and 23 which are made to'move along the length of the crystal by the slotted bar 26 of insulation material which is attached to the pivot rod 21 actuated by the knob 28. The ends of the crystal rest in the notched members 24 and II. 55 Any one of these three fundamental types of holders may be used and the mechanical arrangement for moving the electrodes may be varied to suit the size and external design of the complete unit. v

It is of course obvious that I do not desire to limit this invention to the exact details shown and described except insofar as they are defined by the claims.

What I claim is:

1. A piezoelectric crystal elementadapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform 70 thickness having the principal faces thereof of bowed configuration ground so that different elemental segments thereof have different frequency thickness coefficients.

2. A piezoelectric crystal element adapted to 7; respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal eiement of substantially uniform thickness having the principal faces thereof ground so that different elemental segments of the said crystal element have different frequency thickness coeflicients.

3. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof of bowed configuration ground so that different elemental segments thereof have different frequency thickness coefficients, said principal faces being cut substantially parallel to an electric axis of the mother crystal and at angles between plus or minus 90 degrees with respect to the optic axis.

4. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof of bowed configuration ground so that different elemental segments thereof have different frequency thickness coefficients, said principal faces being cut substantially parallel to an electric axis of the mother crystal and at angles between plus 1'7 degrees and plus 30 degrees with respect to the optic axis.

5. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof of bowed configuration ground so that different elemental segments thereof have different frequency thickness coefficients, said principal faces being cut substantially parallel to an electric axis of the mother crystal and at angles between minus 30 degrees and minus 45 degrees with respect to the optic axis.

6. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof of bowed configuration ground so that different elemental segments thereof have different frequency thickness coefficients, said principal faces being cut substantially parallel to an electric axis of the mother crystal and at angles between plus 10 degrees and minus degrees with respect, to the optic axis.

'7. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof of bowed configuration ground so that diflerent segments of said crystal element have different frequency thickness coeiilcients, a pair of electrodes for said crystal element, at least one of said electrodes being movable over the surface of the corresponding principal face to select different sections of said crystal element for generating electrical oscillations the frequency of which may be continuously varied as said electrode is moved over the different crystal sections.

8. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be varied continuously over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof out to form sections of a cylinder whereby'said crystal element consists of a multiplicity of segments of substantially uniform thickness all having different frequency thickness coefllcients so that said crystal element is responsive to and adapted to generate electrical oscillations the frequency of which may be continuously varied as different ones of said'crystal segments are selected.

9. A piezoelectric crystal element adapted to respond to and generate electrical oscillations i the frequency of which may be varied continuously over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof cut to form sections of a cylinder whereby said crystal element consists of a multiplicity of segments of substantially uniform thickness all having different frequency thickness coefficients so that said crystal element is responsive to and adapted to generateielectrical oscillations the frequency of which may be continuously varied as diflerent ones of said crystal segments are selected. said principal faces being cut substantially parallel to an electric axis of the mother crystal and at angles between plus or minus degrees with respect to the optic axis.

10. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be varied continuously over a wide frequency range, comprising: a piezoelectric crystal elementv of substantially uniform thickness having the principal faces thereof cut,

to form sections of a cylinder whereby said crystal element consists of a multiplicity of segments of substantially uniform thickness all having different frequency thickness coeflicients so that said crystal element is responsive to and adapted to generate electrical oscillations the frequency of which may be continuously varied as different ones of said crystal segments are selected, said principal faces being cut substantially parallel .to an electric axis of the mother crystal and at angles between plus 17 degrees and plus 30 degrees with respect to the optic axis.

1l..A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be varied continuously over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof out to form sections of a cylinder whereby said crystal element consists of a multiplicity of segments of substantially uniform thickness all having different frequency thickness coeflicients so that said crystal element is responsive to and adapted to generate electrical oscillations the frequency of which may be continuously varied as different ones of said crystal segments are selected, said principal faces being cut substantially parallel to an electric axis of the mother crystal and at angles between minus 30 degrees and minus 45 degrees with respect to the optic axis.

12. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be varied continuously over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof cut to 'form sections of a cylinder whereby said crystal element consists of a multiplicity of segments of substantially uniform thickness all having diflerent frequency thickness coefncients so that said crystal element is responsive to and adapted to generate electrical oscillations the frequency of which may be continuously varied as different ones of said crystal segments are selected, said principal faces being cut substantially parallel to an electric axis of the mother crystal and at angles between plus 10 degrees and minus 40 degrees with respect to the optic axis.

13. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof of bowed configuration ground so that diflerent segments of said crystal element have different frequency thickness coefficients, a pair of electrodes for said crystal element, one of said electrodes being in the shape of an elongated roller and being movable over the surface of the corresponding principal face to select different sections of said crystal element for generating electrical oscillations the frequency of which may be continuously varied as said electrode is moved over thediiferent crystal sections.

14. A piezoelectric crystal element adapted to respond to and generate electrical oscillations the frequency of which may be continuously varied over a wide frequency range, comprising: a piezoelectric crystal element of substantially uniform thickness having the principal faces thereof of bowed configuration ground so that diflerent segments of said crystal element have different frequency thickness coefllcients, a-pair of electrodes for said crystal element, said electrodes being roller shaped and means for moving each of said roller shaped electrodes over the surface of the corresponding principal face to select different sections of said crystal element for generating electrical oscillations the frequency of which may be continuously varied as said electrode is moved over the diflerent crystal sections. a

' JOHN M. WOLFSKILL. 

